Numerical and Experimental Validation of the Passive Performance of a Coharmonic Gyro-Multiplier Interaction Region

The azimuthally rippled cavity for a large-orbit, coharmonic gyro-multiplier, designed to operate at the second and fourth harmonics, at frequencies of 37.5 and 75 GHz, respectively, has been numerically and experimentally confirmed to be insensitive to the polarization of quadrupole,
${\text{TE}}_{2,n}$
-like modes, including the second-harmonic operating mode of the multiplier, a cylindrical TE
_2,2
-like waveguide mode. To test the cavity with this mode required the design, construction, and measurement of ripple wall mode converters, converting the cylindrical TE
_2,1
mode into the TE
_2,2
mode. These were designed to operate at a central frequency of ~37.9 GHz, with predicted mode purity of better than 85%, and 3-dB bandwidth of 161 MHz. The constructed converter had a central operating frequency of 37.7 GHz, with
$S$
-parameter measurements used to infer suitable mode purity and an operational 3-dB bandwidth of 50 MHz. This has allowed far-field phase measurements of the corrugated cavity to be conducted, where the orientation of the geometry to the polarization of both the TE
_2,1
and TE
_2,2
modes was shown to have no effect on the dispersion.